In recent years, sunlight energy has attracted attention as an energy source in view of environmental problems.
Methods to convert the light or heat of sunlight into electric energy which is one of the typical easily usable energy forms have been put to practical use.
In these methods, the method to convert the sunlight into electric energy, for example, is a typical example, and a photoelectric conversion element is used for this method.
As photoelectric conversion elements, widely utilized have been those employing inorganic materials, for example, single crystal silicon, polycrystalline silicon, amorphous silicon, cadmium telluride and copper indium gallium selenide. Such a photoelectric conversion element has been widely used for so called “a solar cell”.
In such a solar cell employing a photoelectron conversion element in which an inorganic material is used, there have been problems, for example: a highly purified material of, for example, silicon which is used as a constituting material, is needed, the highly purified material being produced via a high purification process; and the manufacturing cost of the solar cell is high because the cell is manufactured via a large number of complicated processes due to the multi-layered p-n junction structure.
On the other hand, investigation of a photoelectric conversion element using an organic material, which has a simpler structure, has also been advanced.
For example, proposed has been an organic p-n junction type photoelectric conversion element in which a perylene tetracarboxylic acid derivative which is an n-type organic dye and copper phthalocyanine which is a p-type organic dye are joined.
In order to improve the shorter exciton diffusion length and the thinner space charge layer which have been considered as demerits of an organic photoelectric conversion element, the attempts to increase the area of p-n junction by simply laminating organic layers to sufficiently keep the number of organic dyes taking part in the charge separation is achieving its object.
Also, there has been proposed a method in which an n-type electron transporting organic material and a p-type hole transporting polymer are mixed in a layer to form a composite to drastically increase the area of p-n junction, whereby charge separation is carried out in whole the layer. Furthermore, a photoelectron conversion element employing a conjugated polymer as a p-type conductive polymer in which fullerene is mixed as an electronic conducting material is proposed.
Although these photoelectric conversion elements are raising the characteristics gradually, a stable performance while keeping a high conversion efficiency has not been fully obtained.
In 1991, as the results of large number of precise experiments on a photoelectric current sensitized by a dye absorbed on porous titanium dioxide, Gratzel succeeded to produce a photoelectric conversion element which stably exhibits a high conversion efficiency by providing a sufficient area for charge separation or a sufficient number of molecules taking part in charge separation employing a porous titanium oxide (for example, refer to Non-Patent Document 1).
In this photoelectric conversion element, there repeated is a cycle in which a dye adsorbed on the porous titanium oxide is photo-excited, first, to inject electrons to the titanium oxide while the dye is formed into a cation of the dye, and the cation of the dye receives an electron migrated from the counter electrode through a hole transfer layer.
Combined with the stable nature of titanium dioxide, this photoelectric conversion element exhibits an excellent reproducibility, and the base of development has widely spread. This photoelectric conversion element is referred to as “a dye-sensitized solar cell”, and has attracted high expectations and attentions.
This method has many advantages, for example, an inexpensive metal compound semiconductor, such as titanium dioxide, can be used without refinement to high purity, whereby cheap semiconductor materials can be used, and, further, light of a wide range of visible light can be utilized, whereby the sun light having a wide range of visible light can be effectively converted into electric energy.
However, this method has problems, for example, an expensive ruthenium complex is necessary because a ruthenium complex is used in the photoelectric conversion layer, ruthenium having a resource restriction.
As a further problem of this method, an additional mechanism which retains the electrolyte liquid or iodine in the element or prevents those from flowing out or dissipation since such a solar cell is operated employing an electrolyte liquid, as aforementioned.
Typical examples of an electrochemical element having an electrolyte liquid include a lead-acid battery and a lithium battery. Even in these electrochemical elements fabricated in a compact module, the electrolyte is not fully recovered and recycled, and it is obvious that a secondary problem may be induced when dissipated chemicals are newly accumulated in the environment.
Development of an all solid dye-sensitized solar cell free from these electrolyte problems while keeping the advantage of the dye-sensitized solar cell is progressing.
In this field, a dye-sensitized solar cell employing an amorphous organic hole transport material, or a dye-sensitized solar cell employing copper iodide as a hole transport material has been known. However, not fully sufficient photoelectric conversion efficiency has been obtained due to the lower conductivity of the hole transport material.
As a hole transport material exhibiting a higher conductivity, polythiophene materials are raised as typical examples, and an all solid dye-sensitized solar cell using PEDOT as a hole transport material has been reported (for example, refer to Patent Documents 1 and Non-Patent Document 2).
However, since PEDOT has absorption in a visible light range (400-700 nm), loss in light absorption by the dye occurs, and the photoelectric conversion efficiency has not been fully enough. Moreover, since PEDOT suffers from deterioration due to light exposure, the durability of PEDOT has not been fully enough.
Patent Document 1 Japanese patent publication application (hereafter referred to as JP-A) No. 2003-317814
Non-Patent Document 1 B. O'Regan and M. Gratzel: Nature, 353, 737 (1991)
Non-Patent Document 2 J. Xia and N. Masaki, M. Lira-Cantu, Y. Kim, K. Jiang and S. Yanagida: Journal of the American Chemical Society, 130, 1258 (2008)